Heat a medium-sized pan (ideally cast-iron) over medium-high heat, and add olive oil to prevent sticking. Add scallions and grill, pressing them flat with a spatula, until lightly charred, about 3 minutes.

Transfer scallions to a work surface, and cut into 1/2″ pieces. In a small bowl, mix scallions with lime juice and serranos, and season with salt and pepper to taste.

Season the steak with salt and pepper, and grill over medium-high heat, turning once, until lightly charred on the outside and medium rare within, about 6 minutes per total. Transfer to a carving board, cover, and let rest for about 5 minutes. Slice the meat against the grain, and place on Croccantini, then top with scallion-serrano mixture and serve warm.